1. Field of the Invention
This invention relates in general to portable wireless communications systems. In particular, the invention relates to a digital circuit clocking system for providing a low power sleep mode in a wireless communications system, such as a cordless telephone.
2. Background Art
Devices incorporating wireless communications techniques are becoming increasingly prevalent in modern society. Moreover, most such devices are now incorporating digital communications techniques to provide greater reliability, enhanced functionality, more efficient bandwidth utilization and improved communications quality. However, system designers are increasingly being asked to meet demands of increased functionality while simultaneously reducing the size and cost of their products.
One communications technique that is commonly used to meet these demands is Frequency Hopping Spread Spectrum (“FHSS”) technology. A FHSS transceiver operates by rapidly changing its tuned carrier frequency over time in a known pattern, called the hop sequence or hop pattern. By using different hop sequences, multiple users can communicate simultaneously over differing communications channels all within a common frequency bandwidth. FHSS offers improved communications quality over other solutions in noisy environments because a source of interference at any given frequency affects communications only momentarily. Thus, when the number of noisy channels in the hop sequence is relatively low, the resultant degradation in data throughput, and hence communication quality, is often minimal. Also, many FHSS systems provide for dynamic allocation of frequencies in the hopping sequence, such that static sources of interference can be detected and avoided entirely.
Many FHSS systems which include portable, battery-powered transceivers as communications devices also implement a “sleep mode” feature to reduce power consumption, thereby extending battery life. A sleep mode typically operates by depowering some circuits within the device and reducing the clocking rate of various other digital circuitry during periods of inactivity, thereby reducing electrical power consumption. Such systems then “wake up” periodically to briefly determine whether active communications are required. For example, in a cordless telephone system, the handset may reside in a sleep mode for a majority of the time. The handset wakes up periodically to determine whether a call is being received, or whether the user has indicated a desire to use the system by pressing a button on the handset keypad. If activity is required or requested, the handset enters into and remains in its awake mode, and operates according to its intended functionality. When no activity is required or requested, the handset returns to its sleep mode, thereby resuming its state of reduced power consumption. Battery life can thus be maximized by minimizing the time spent in the awake mode checking for activity.
One aspect of implementing such a sleep mode is providing a clock signal of substantially reduced frequency for clocking of digital circuits. However, conventional FHSS systems face several tradeoffs in implementing a sleep mode with reduced clocking rates. Oscillators are often based upon ceramic or crystal oscillator circuits, which provide a very high degree of accuracy—typically in excess of 0.01%. However, crystal or ceramic oscillators that operate at frequencies sufficiently low to implement a low-power sleep mode are typically large in size and expensive. Thus, low frequency crystals or ceramics are highly disadvantageous for implementing a sleep mode in a compact product for price-sensitive, consumer applications.
Many non-frequency hopping systems utilize a simple and inexpensive RC oscillator (based upon a parallel resistor-capacitor combination) to drive circuits during low-frequency sleep modes. The operating frequencies of such oscillators are typically highly inaccurate, often varying by 10% or more with component tolerances, battery voltages, component temperature, aging, and other factors. This inaccuracy is often inconsequential in systems implementing communications protocols with fixed carrier frequencies, because a constant, fixed frequency broadcast signal can be transmitted with which the portable unit can synchronize upon awaking. However, when a portable unit implementing a frequency hopping system is subjected to timing inaccuracies during its sleep mode, critical timing may drift to the point that the portable unit occupies a different position in the hop sequence than the base unit when the portable unit wakes from the sleep mode. The portable unit may thus proceed to tune its receiver according to the hop sequence, but at a different position in the sequence compared to the base, such that upon returning to the awake mode the portable unit receiver is tuned to a different frequency than the base unit transmitter by virtue of being at different positions in the hop sequence (i.e. the portable unit has lost synchronization). When this occurs, communications typically fail, and system timing must be physically reset before communications can resume.
Several techniques can be implemented to combat such loss of synchronization due to sleep mode induced frequency drift. Since the extent of timing drift is proportional to the duration of sleep time as well as the difference in frequency between the base and handset oscillators, one solution is to reduce the duration of time during which the handset remains in its sleep mode, between periods of waking and reacquiring timing synchronization. However, this increases the ratio of wake time to sleep time, thereby increasing the average power consumption, and reducing battery life.
Other techniques that can be used to combat problems attributable to sleep mode time drift involve altering the hopping pattern of the communications system. For example, in a “marking time” mode the base and portable units hop on a single, pre-arranged frequency so that when the handset awakens, it detects the base unit's query on the pre-arranged frequency and responds thereto. However, if interference is present on the prearranged frequency, subsequent communications can be blocked such that the handset is unable to reacquire synchronization and resume regular communications. Other strategies involve implementing a special slow-hopping pattern whereby the base unit and handset increase the time during which they dwell on each frequency in the hopping pattern, and/or they restrict their hopping to a subset of available working frequencies. These strategies suffer from an increase in the time required to recapture synchronization, thereby degrading system responsiveness. Furthermore, the altered hopping pattern techniques can cause problems in complying with FCC regulations and/or other telecommunications standards, in that such specifications often regulate hop timing, hop sequence size, and/or channel randomization. Finally, systems capable of supporting multiple handsets can be difficult, if not impossible, to implement with such altered standby hopping patterns because some handsets may require that the base implement an altered hopping pattern while other handsets are actively communicating with the base via the regular hopping pattern.